1. Field of Invention
The present invention generally relates to high-efficiency, linear illumination sources and linear illumination systems which have enhanced output irradiance and radiance. Irradiance is defined as the light flux per unit area and can be expressed, for example, in units of watts per square centimeter (W/cm.sup.2). Radiance is the brightness of the light. Radiance can be expressed, for example, in units of watts per square centimeter per steradian (W/(cm.sup.2.multidot.steradian), where a steradian is the unit of solid angle.
For many applications, an illumination source with a narrow output opening and high output efficiency is preferred. Such a source is commonly constructed using an aperture lamp with an internal slit aperture built into the lamp structure. However, an aperture lamp generally has lower light emission than a conventional lamp due to increased light absorption inside the lamp and due to a reduction in the surface area of the phosphor coating. It would be highly desirable to have an improved narrow illumination source that is more efficient than a lamp with an internal slit aperture.
For applications such as, for example, optical scanners and photocopiers, a linear illumination system with high output irradiance is desired in order to illuminate a narrow strip of the area being scanned or photocopied. The illumination assembly for such a device commonly consists of a bare linear light source, an aperture lamp, or a lamp partially surrounded with a specular reflector. A specular reflector is a mirror-like reflector with a smooth surface and has the property that the angle of light incidence equals the angle of reflection, where the incident and reflection angles are measured relative to the direction normal to the surface. An improved linear illumination system which has higher output irradiance would be advantageous.
For certain other applications such as flat panel displays, an illumination system having a very shallow thickness is highly desirable. Such systems are commonly configured with one or more illumination sources, a waveguide or light pipe for collecting and distributing the light from the illumination sources, and additional scattering, reflecting, or collimating elements for extracting the light from the waveguide. A significant depth savings can be achieved by coupling the illumination sources through the edge of the waveguide. The amount of light extracted from the system is proportional to the number of reflections or scattering events that occur within the waveguide, the number being inversely proportional to the thickness of the waveguide. To obtain maximum light output, a thin waveguide is preferable. However, this results in waveguide edges having a small surface area, limiting the size of the illumination source that can directly adjoin the edge of the waveguide. On the other hand, if the surface area of the waveguide edge is increased, the extraction efficiency of the waveguide will decrease. It would be highly desirable to utilize a thin waveguide yet provide the maximum illumination source input. Therefore, a highly-efficient, linear illumination source with high output irradiance and radiance from a narrow opening is needed.
2. Description of the Prior Art
It is well-known that it is possible to use tubular fluorescent lamps having an internal slit aperture in order to concentrate and direct the emitted light into a narrow angular range. Two types of aperture lamps with internal slits are in general use. The first type is shown in cross section as aperture lamp 10 in FIG. 1. The lamp is composed of a hollow glass tube 12 having a phosphor coating 14 on the entire inside surface except in one narrow region 16 subtending angle 18. The center of the tube is filled with a mixture of gases which, when excited by an electric current supplied by electrodes (not shown) at the ends of the tube, emits ultraviolet light. The ultraviolet light, in turn, strikes the phosphor coating 14 and is converted to visible light. A typical phosphor coating is also a diffuse reflector. Note that a diffuse reflector is a reflector that scatters incident light into a range of angles. Diffuse reflectors typically have high reflectivity only when the reflective coating is relatively thick (e.g. about 0.15 mm or greater). The reflective phosphor coating on the inside of an aperture lamp is, by necessity, significantly thinner than 0.15 mm resulting in poor reflectivity (on the order of 60-80%). Most of the light not reflected by the phosphor is transmitted through the coating. By placing an aperture, in this case gap 16, in the phosphor coating, light can be directed preferentially out the aperture. However, due to loss of some of the light through the phosphor coating, the effectiveness of this type of aperture lamp is significantly reduced.
A second type of lamp having an internal aperture and known to those skilled in the art is shown in FIG. 2 as aperture lamp 50. The lamp has a glass tube 52. Inside the glass tube is a phosphor coating 54 and an additional reflective coating 56. There is an aperture opening 58 through both the phosphor coating 54 and reflective coating 56 which subtends angle 59 and which allows light to escape preferentially in one direction.
There are six significant problems associated with the internal aperture lamps 10 and 50 shown in FIGS. 1 and 2. First, the phosphor and reflective coatings must be very thin and the selection of coating materials is very limited so as not to interfere with the operation of the lamp. No organic materials are possible for an internal coating because any outgassing from the organic material or decomposition of the organic material from the effects of ultraviolet light would lower the efficiency of the lamp. Second, because of the restrictions on coating materials, the reflectivity of the coatings is not as high as desired. Third, a significant amount of ultraviolet light generated inside the lamp is wasted due to absorption by the glass tube in the area without the phosphor coating. Fourth, a more expensive glass must be used to make these types of aperture lamps in order to reduce ultraviolet light induced discoloration and loss of light transmission of the glass in the area of the aperture. Fifth, because the area of the internal lamp surface which is covered by the phosphor coating is reduced by the area which includes the aperture, there is a corresponding reduction in the efficiency of converting electrical power to light energy. Sixth, internal aperture lamps are more difficult to manufacture than conventional lamps and therefore are more expensive. Such deficiencies contribute to reduced efficiency and higher costs for aperture lamps compared to regular lamps without internal apertures.
Accordingly, there are now provided with this invention improved linear illumination sources which utilize external, highly reflective enclosures incorporating one or more linear openings in order to achieve improved source efficiency, output irradiance and output radiance. Such improved illumination sources may be combined with additional optical elements to produce more complex illumination systems. Additional objects of the present invention will become apparent from the following description.